Prevention of audio pop in a digital audio device

ABSTRACT

An electronic audio device with a digital audio output channel in which an amplifier output voltage is gradually ramped up and down to avoid causing a popping sound when the device is turned on and off. This is accomplished without employing any additional hardware, by incrementally changing a digital input word applied to a digital audio source, such as a DSP, so as to gradually change the amplifier output voltage between a minimum, such as zero volts, and a DC working voltage. On powering up, the amplifier is only turned on after the digital word is applied, but while it still results in a minimum amplifier output, and on powering down the amplifier is turned off after it&#39;s output has been ramped down, but before removing the digital input word. Sources and output channels can also be switched over by powering down, and then powering up, following the same method.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to preventing audio pop in an electronic audio device. More particularly, the invention relates to preventing audio pop in a digital audio circuit, such as in a cellular telephone.

[0003] 2. Background of Related Art

[0004] An electronic device with an audio output, such as for example a cellular telephone, may include one or more audio channels. FIG. 5 shows a conventional audio output channel in a digital electronic audio device.

[0005] In particular, as shown in FIG. 5, a circuit such as that found in conventional cellular telephones includes an audio output channel driven by a digital audio source (e.g., a digital signal processor (DSP)) 100. The digital output signal corresponding to the relevant audio is output from the DSP 100 and converted to an analog signal by a digital-to-analog (D/A) converter 110 (e.g., a COder/DECoder (codec)), followed by an analog filter 120, and an amplifier 130. A speaker or headset load 150 is AC coupled to the output of the amplifier 130, represented in FIG. 5 by a capacitor 140.

[0006] However, when such a conventional audio output channel is powered on, a DC component of the output voltage of the amplifier 130 suddenly increases from, e.g., zero volts to it's DC working level of voltage. This sudden voltage change, when coupled through the AC coupling capacitor 130, causes the AC coupling capacitor 340 to be charged rapidly, thus having a tendency to cause an annoying popping sound at the audio load 150 (e.g., speaker or headset), particularly if powered on at the peak of a loud waveform. Similarly, even when the audio circuit is turned off and the supply voltage is removed from the audio amplifier stage 130, the DC output voltage level suddenly decreases, rapidly discharging the AC coupling capacitor 140, thus causing a further popping sound to the listener through the audio load 150. In extreme cases, the cumulative effect of such loud popping sounds may even be harmful to the hearing of the listener.

[0007] Carroll et al. show in U.S. Pat. No. 6,157,726 a previous attempt to solve the above problem, reproduced in FIG. 6 of this application.

[0008] In particular, as shown in FIG. 6 herein, the output of the audio amplifier 130 is AC coupled to a parallel combination of two paths, each including a switch 570, 580. One path includes the audio load 150 and the switch 580, while the other path includes a dummy load 560 and the other switch 570. The audio load 550 may be a speaker, headset, or the like, whereas the dummy load 560 may be simply a resistor or other resistive device.

[0009] In the operation of the circuit of FIG. 6, the first analog switch 570 is closed and the second analog switch 580 is open when the supply voltage is initially applied to amplifier 530. The rising DC output voltage from amplifier 530 therefore charges the AC coupling capacitor 540, but the capacitor 540 is connected to the dummy load 560, and so no popping sound is heard at the load 550. Then, after the AC coupling capacitor 540 has been charged up, the first analog switch 570 is opened and the second analog switch 580 is closed, connecting the amplifier 530 to the load 550 via the AC coupling capacitor 540. Before the supply voltage to the amplifier 530 is turned off, the second analog switch 580 is re-opened and first analog switch 570 is re-closed, so that the AC coupling capacitor is discharged through the dummy load 560, thus avoiding a popping sound heard by the listener at the actual audio load 550.

[0010] However, the circuit of FIG. 6 is somewhat disadvantageous in that it requires not only additional components in the audio path itself (e.g. two switches 570, 580 and a dummy load 560), but also some form of control circuitry, including circuitry needed to sense when the audio output channel is being turned on or off.

[0011] There is a need for a technique and apparatus to eliminate a popping sound without the introduction of additional and/or complex circuitry.

SUMMARY OF THE INVENTION

[0012] A method and apparatus for applying power to a digital audio channel in accordance with the principles of the present invention comprises applying a digital input signal to the digital audio channel. The digital input signal initially has a value such that a DC output voltage of the digital audio channel is at a minimum. The value of the digital input signal is gradually increased independent of an input audio signal such that the DC output voltage increases gradually until the DC output voltage is equal to a working voltage. Then the input audio signal is passed through the digital audio channel.

[0013] A method and apparatus for removing power from a digital audio channel in accordance with another aspect of the present invention comprises halting an input audio signal from passing through the digital audio channel. A value of a digital input signal is gradually decreased from a point of the halted input audio signal such that a DC output voltage of an amplifier of the digital audio channel decreases gradually until the DC output voltage is at a minimum.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Features and advantages of the present invention will become apparent to those skilled in the art from the following description with reference to the drawings, in which:

[0015]FIG. 1 is a block diagram of an audio output channel for digital audio.

[0016]FIG. 2 is a flow chart of a method according to the present invention.

[0017]FIG. 3 is a further flow chart of a method according to the present invention.

[0018]FIG. 4 is a block diagram of an audio output channel for digital audio according to an alternative embodiment of the invention.

[0019]FIG. 5 shows a conventional audio output channel in a digital electronic audio device.

[0020]FIG. 6 shows a conventional audio circuit as shown in U.S. Pat. No. 6,157,726.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0021] A block diagram of a digital audio output channel for an electronic audio device, such as, for example, a cellular telephone, is shown in FIG. 1.

[0022] In particular, as shown in FIG. 1, a digital audio output channel is driven by a digital audio source 300, which may be, for example, a Digital Signal Processor (DSP). The digital audio output channel also includes a digital-to-analog (D/A) converter 310, an analog filter 320, and an amplifier stage 330. An audio output load 350 is AC coupled via an AC coupling capacitor 340. The audio output load 350 may be any appropriate audio output transducer, e.g., a loudspeaker or a headset. AC coupling capacitor 340 serves in the usual fashion to block the DC component of the dynamic analog output signal from the audio amplifier 330.

[0023] Importantly, the audio output circuit in accordance with the principles of the present invention further includes an on/off ramping controller to control the output signal from the audio amplifier 330 to any desired DC level. In the disclosed embodiments, the on/off ramping controller 305 ramps either linearly or non-linearly between a DC start value (X) stored in a first register or similar memory location 306, and a desired DC working value (Y) stored in a second register or similar memory location 307.

[0024] The DC start value X and/or the DC working value Y may be empirically determined based on desirous results. Alternatively, the DC start value X and/or the DC working value Y may be pre-set to values such as X=0 volts and Y=0.5 times the maximum DC level output from the audio amplifier 330. Other determinations of the DC start value X and DC working value Y are possible and within the scope of the present invention. For instance, a moving average of the DC level output from the audio amplifier 330 may be measured over a period of time and used as a basis for the determination of the DC working value Y. Also, the DC working value Y may be measured during a quiet period.

[0025] It will be appreciated by one skilled in the art that the audio output channel configuration shown in FIG. 1 is but one exemplary arrangement, and that many other circuit variations are possible and encompassed by this disclosure.

[0026] In conventional digital audio circuits, the AC coupling capacitor stores a substantial electrical charge in normal operation, giving rise to the possibility of a popping sound to the listener.

[0027] Thus, in accordance with a preferred embodiment of the present invention, additional circuitry such as is required by Carroll et al. in U.S. Pat. No. 6,157,726 is eliminated by employing the on/off ramping controller 305, including references to ramp from a DC start value X to a DC working value Y, before allowing full audio operations of the digital audio circuit.

[0028] The length of the ramp in time may be adjustable, and is preferably sufficiently short to have little if any effective delay as perceived by the listener, yet allows sufficient time for a smooth transition of the DC output of the audio amplifier 330 to a suitable working voltage level.

[0029] The on/off ramping controller 305 may be a separate element, or preferably integrated within a processor already resident in the relevant circuit (e.g., a digital signal processor). The on/off ramping controller 305 gradually ramps up as the digital audio path is turned on, and gradually ramps down the output voltage of the audio amplifier stage 330, preferably under the control of software.

[0030]FIG. 2 shows a flowchart of an exemplary method of turning an electronic audio device on, according to a preferred embodiment of the present invention.

[0031] In particular, as shown in step 100 of FIG. 2, all elements in the digital audio channel other than the amplifier stage 330 are fully powered up, i.e. the digital audio source 300, the D-A converter 310 and the analog filter 320 are each powered up, leaving the amplifier stage 330 in an off state. Then, in step 110, a digital word “X” is applied to the digital audio source 300, where the digital word “X” corresponds to an output voltage of zero volts from the amplifier stage 330. Next, in step 120, the amplifier stage 330 is powered up. At that point in time, the output voltage of the amplifier stage 330 will be zero volts. Then, in step 130, the digital word applied to the digital audio source 300 is changed gradually from “X” to “Y”, where the digital word “Y” corresponds to the normal DC working voltage being present at the output of the amplifier stage 330. This gradual change in the value of the digital word generates a voltage ramp at the output of the amplifier stage 330. The slope of this voltage ramp is adjusted to take into account the total combined group delay of any internal filters and of the AC coupling capacitor 340 and the load 350. The digital audio channel is then powered up and is ready for use.

[0032]FIG. 3 is a flowchart of an exemplary method of turning the electronic audio device off according to a preferred embodiment of the invention.

[0033] In particular, as shown in step 200 of FIG. 3, the digital word is gradually changed from it's last state to “X”. Then, in step 210, the amplifier stage 330 is powered off.

[0034] In step 220, the digital input word “X” is finally removed. Of course, the digital value X may be left in place at all times when the amplifier 330 is powered on and unused. Then, when it is powered down, the output signal will fall only between the X level and 0 or ground (presuming that the X value is non-zero).

[0035] In step 230, the digital audio source 300, the D-A converter 310 and the analog filter 320 are each powered down, either simultaneously or in sequence. At this point, the digital audio channel is powered down.

[0036] The shape of the voltage ramp employed in the above methods is not necessarily linear, but may be any shape, for example exponential or other non-straight-line method.

[0037] The digital word corresponding to the DC start value “X” need not necessarily correspond to an output voltage of zero volts, but rather may correspond to a non-active level of the audio signal at the output of the audio amplifier 330.

[0038] A digital audio output channel according to an alternative preferred embodiment of the invention is shown in FIG. 4.

[0039] In particular, FIG. 4 shows the digital audio output channel driven by a digital audio source 400, a D/A converter 410, an analog filter 420, an analog switch 490, an amplifier stage 430, an AC coupling capacitor 440, and an audio load 450, as shown and described with respect to similar elements in FIG. 1. However, in this particular embodiment, the input to the audio amplifier stage 430 is gated by an analog switch 490 (as opposed to the output switching described in U.S. Pat. No. 6,157,726). The analog switch 490 may be controlled by any appropriate controller, e.g., DSP, or even by the digital source 400 itself.

[0040] DC input ramping in a digital audio channel in accordance with the principles of the present invention may be combined with otherwise conventional techniques, e.g., with analog switching as shown in U.S. Pat. No. 6,157,726.

[0041] The above embodiments may also be adapted to situations where a digital audio source is switched from one digital audio channel to another, or where the input of a digital audio channel is switched between a plurality of digital audio sources. To achieve such a changeover, the digital audio channel in use is first powered down, e.g., according to the method of FIG. 3, then the existing digital audio source may be disconnected from the existing digital audio channel. Next, the appropriate digital audio source and digital audio channel may be connected together, and finally this digital audio channel is powered up, e.g., according to the method of FIG. 2.

[0042] It will be appreciated by those skilled in the art that the invention may be applicable to a wide variety of electronic equipment employing digital audio, including not only cellular telephones but also DVD video players, music CD players, computer sound cards, DAT tape recorders and communications receivers employing DSP.

[0043] While the invention has been described with reference to the exemplary embodiments thereof, those skilled in the art will be able to make various modifications to the described embodiments of the invention without departing from the true spirit and scope of the invention. 

What is claimed is:
 1. A method of applying power to a digital audio channel, comprising: applying a digital input signal to said digital audio channel, said digital input signal initially having a value such that a DC output voltage of said digital audio channel is at a minimum; gradually increasing said value of said digital input signal independent of an input audio signal such that said DC output voltage increases gradually until said DC output voltage is equal to a working voltage; and then passing said input audio signal through said digital audio channel.
 2. The method of applying power to a digital audio channel according to claim 1, further comprising: turning on an audio amplifier stage in said digital audio channel after said step of applying said digital input signal and before said step of gradually increasing said value of said digital input signal.
 3. The method of applying power to a digital audio channel according to claim 2, wherein said step of turning on said audio amplifier stage comprises: applying power to said audio amplifier stage.
 4. The method of applying power to a digital audio channel according to claim 2, wherein said step of turning on said audio amplifier stage comprises: connecting said audio amplifier stage to an audio transducer.
 5. The method of applying power to a digital audio channel according to claim 1, wherein: said minimum is a potential of a ground reference of said audio amplifier stage.
 6. A method of removing power from a digital audio channel, comprising: halting an input audio signal from passing through said digital audio channel; and gradually decreasing a value of a digital input signal from a point of said halted input audio signal such that a DC output voltage of an amplifier of said digital audio channel decreases gradually until said DC output voltage is at a minimum.
 7. The method of removing power from a digital audio channel according to claim 6, further comprising: turning off said amplifier after said step of gradually decreasing said value of said digital input signal.
 8. The method of removing power from a digital audio channel according to claim 7, wherein said step of turning off said amplifier comprises: removing power from said amplifier.
 9. The method of removing power from a digital audio channel according to claim 7, wherein said step of turning off said amplifier comprises: disconnecting said amplifier from an audio transducer.
 10. The method of removing power from a digital audio channel according to claim 6, wherein: said minimum is a potential of a ground reference of said amplifier.
 11. A digital audio channel of a wireless telephone, comprising: means for applying a digital input signal to said digital audio channel, said digital input signal initially having a value such that a DC output voltage of said digital audio channel is at a minimum; means for gradually increasing said value of said digital input signal independent of an input audio signal such that said DC output voltage increases gradually until said DC output voltage is equal to a working voltage; and then means for passing said input audio signal through said digital audio channel.
 12. The digital audio channel of a wireless telephone according to claim 11, further comprising: means for turning on an audio amplifier stage in said digital audio channel after said step of applying said digital input signal and before said step of gradually increasing said value of said digital input signal.
 13. The digital audio channel of a wireless telephone according to claim 12, wherein said means for turning on said audio amplifier stage comprises: means for applying power to said audio amplifier stage.
 14. The digital audio channel of a wireless telephone according to claim 12, wherein said means for turning on said audio amplifier stage comprises: means for connecting said audio amplifier stage to an audio transducer.
 15. The digital audio channel of a wireless telephone according to claim 11, wherein: said minimum is a potential of a ground reference of said audio amplifier stage.
 16. Apparatus for removing power from a digital audio channel, comprising: means for halting an input audio signal from passing through said digital audio channel; and means for gradually decreasing a value of a digital input signal from a point of said halted input audio signal such that a DC output voltage of an amplifier of said digital audio channel decreases gradually until said DC output voltage is at a minimum.
 17. The apparatus for removing power from a digital audio channel according to claim 16, further comprising: means for turning off said amplifier after said means for gradually decreasing said value of said digital input signal decreases said value to said minimum.
 18. The apparatus for removing power from a digital audio channel according to claim 17, wherein said means for turning off said amplifier comprises: means for removing power from said amplifier.
 19. The apparatus for removing power from a digital audio channel according to claim 17, wherein said means for turning off said amplifier comprises: means for disconnecting said amplifier from an audio transducer.
 20. The apparatus for removing power from a digital audio channel according to claim 16, wherein: said minimum is a potential of a ground reference of said amplifier. 